Papers by Author: David Wexler

Abstract: Insulated rail joints (IRJs) are a primary component of the rail track safety and signalling systems. Rails are supported by two fishplates which are fastened by bolts and nuts and, with the support of sleepers and track ballast, form an integrated assembly. IRJ failure can result from progressive defects, the propagation of which is influenced by residual stresses in the rail. Residual stresses change significantly during service due to the complex deformation and damage effects associated with wheel rolling, sliding and impact. IRJ failures can occur when metal flows over the insulated rail gap (typically 6-8 mm width), breaks the electrically isolated section of track and results in malfunction of the track signalling system. In this investigation, residual stress measurements were obtained from rail-ends which had undergone controlled amounts of surface plastic deformation using a full scale wheel-on-track simulation test rig. Results were compared with those obtained from similar investigations performed on rail ends associated with ex-service IRJs. Residual stresses were measured by neutron diffraction at the Australian Nuclear Science and Technology Organisation (ANSTO). Measurements with constant gauge volume 3x3x3 mm³ were carried in the central vertical plane on 5mm thick sliced rail samples cut by an electric discharge machine (EDM). Stress evolution at the rail ends was found to exhibit characteristics similar to those of the ex-service rails, with a compressive zone of 5mm deep that is counterbalanced by a tension zone beneath, extending to a depth of around 15mm. However, in contrast to the ex-service rails, the type of stress distribution in the test-rig deformed samples was apparently different due to the localization of load under the particular test conditions. In the latter, in contrast with clear stress evolution, there was no obvious evolution of d₀. Since d₀ reflects rather long-term accumulation of crystal lattice damage and microstructural changes due to service load, the loading history of the test rig samples has not reached the same level as the ex-service rails. It is concluded that the wheel-on-rail simulation rig provides the potential capability for testing the wheel-rail rolling contact conditions in rails, rail ends and insulated rail joints.

Abstract: Advanced materials manufacturing methods require clean, non-polluting, high speed and precise processes, and should result in highly reliable final products. However, traditionally, many functional materials are synthesized by slow reaction techniques that are both energy and time consuming. In such cases there is strong demand for more appropriate materials processing methods that could offer increased rapid reaction rates and energy efficiencies, and be environmentally safe. Electric discharge assisted mechanical milling (EDAMM) is a new and exciting materials processing technique which combines the attributes of conventional mechanical and mechanochemical milling with the several additional processing effects which can be generated by the simultaneous application of electric discharges. It is shown that EDAMM can be applied to synthesize a range of functional materials in a matter of minutes, rather than the hours or days required using traditional techniques. This presentation provides an overview of recent developments in the EDAMM method and its application to rapid materials processing, and the synthesis of certain functional materials. In this report, we demonstrate the versatility of EDAMM by; (i) synthesis of hard materials, (ii) synthesis of functional oxides used in electronic, magnetic and optical applications, (iii) rapid reduction reactions including extraction of metals from oxides and sulfides and (iv) synthesis of fine metallic and nonmetallic powders and (v) formation of nano- fragments, including carbon nanoribbons and iron oxide nanorods.

Abstract: The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Abstract: The effect of boron on the WC morphology and on the grain size of binders in sub micron WC composites containing Fe60Al40 and Ni3Al binders was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composites were prepared under uniaxial hot pressing of milled powder samples at 1500 °C in inert argon atmosphere. Doped aluminides with boron levels ranging from 0 to 0.1 wt% were used as the binders. It was found that the microstructural characteristics of boron doped aluminide WC composites were similar to those of hot pressed WC-Co and commercial grade WC-10wt%Co (H10F) hardmetals. The contiguity of WC particles (WC/WC contact) and the grain sizes of aluminides decreased and the extent of faceting of tungsten carbide increased in the aluminide tungsten carbide composites in presence of boron.

Abstract: The hardness and indentation fracture toughness of sub micron WC composites based on aluminide and cobalt binders were investigated. Doped Fe60Al40 and Ni3Al alloys with boron levels ranging from 0 to 0.1 wt%, were used as the aluminide binders. The composite materials were processed by uniaxial hot pressing of milled powder samples at 1500 °C under argon atmosphere. The hardness of WC-40vol%(FeAl-B) was found to be higher than that of WC-40vol%(Ni3Al-B), and it approached to the hardness level of the commercial grade of WC-10wt%Co (H10F). The fracture toughness of both WC-40vol%(FeAl-B) and WC-40vol%(Ni3Al-B) cermets was higher than that of WC-40vol%Co and the toughness increased with increasing boron content. It is believed that boron addition to the aluminide binders leads to improvement in the fracture toughness of the intermetallic matrix composites as a result of increase in the ductility and toughness of the aluminides and also due to increase in WC solubility in the aluminide binders in presence of boron.

Abstract: Electric discharge assisted mechanical milling using a 50 Hz power supply has been used to produce a range of fine and nanostructural products, including nanocrystalline aglomerates and individual nano-particles and nano-fragments. Processing variables include; starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims of this preliminary investigation were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles, and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological, and phase changes induced by kHz discharge milling were characterised by x-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nano-particles, agglomeration of powder particles, particle melting and/or sintering, and chemical reaction induced by mechanoprocessing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4%H2 resulted in a range of products including; graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanotubes. hematite resulted in partial reduction to magnetite and FeO,and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co-WC resulted in products including; micron and submicron fracture products, nanostructural regions of Co and WC, and carbon rich nanorods and nanotubes.

Abstract: The Zincalume hot-dip coating process is a well-established technique for excellent corrosion protection of steel products. This paper describes the study of two intermetallics based on the Fe- Al-Si-Zn alloy system, α-AlFeSi and α-AlFeSi(+Zn), pertinent to the Zincalume process. These intermetallics are difficult to characterize in-situ due to their formation at high temperature and because they form as very thin intermetallic layers on a steel substrate, which inhibits accurate quantitative analysis. Controlled magneto-ball milling and hot-pressing have been employed in an attempt to synthesise these intermetallic compounds. Magneto-ball milling, under a He atmosphere, was conducted in shearing mode to enable controlled milling of elemental powders, namely Zn and Al, without the excessive cold-welding often associated with milling of ductile powders with high coefficients of surface friction. XRD analysis indicated that uniform mixtures of highly reactive fine-structured powders were produced, as indicated by diffuse elemental peaks of low intensity. Hot-pressing was utilized to sinter the powders into compact intermetallic compounds without sintering aids. The intermetallics were characterized by DTA, XRD and elemental contrast mapping performed on an SEM with EDS.

Abstract: In this paper both electric discharge assisted milling [1, 2] and conventional mechanosynthesis techniques were applied to investigate the effects of milling conditions on the fracture and agglomeration of amorphous CoSiB ribbons produced by planar flow casting. The effect of spark energy on particle shape and size produced by discharge milling was studied. Conventional milling in inert atmosphere for extended periods generally leads to the formation of porous powder particle aggregates, each particle comprised of small amorphous or, after extended milling times, nanocrystalline elements. The mechanism of agglomeration was believed to originate from repeated fracture, deformation and cold welding of individual ribbon elements. In contrast to conventional milling, spark discharge milling was found to induce the formation of predominantly sub-micron single particles of amorphous powder. The morphology of individual particles varied from sub-micron irregular shaped particles to remelted particles, depending on selection of vibrational amplitude during discharge. For high vibrational amplitudes and high energy input a wider range of particles as produced. These included sub-micron particles, remelted particles and welded agglomerates, and nano-sized particles produced as a fume and collected during discharge milling under flowing argon. These results combined with observations that most re-melted particles produced by discharge milling were also amorphous confirmed that extremely high heating and cooling rates are associated with discharge milling of metals. They also confirm the potential of electrical discharge milling as a new route for the synthesis of ultrafine and nanosized powder particles from amorphous ribbon, for possible processing into 3-D shapes.